Polyester has long been recognized as a desirable material for textile applications. The basic processes for the manufacture of polyester are relatively well known and straightforward, and fibers from polyester can be appropriately woven or knitted to form textile fabric. Polyester fibers can be blended with other fibers such as wool or cotton to produce fabrics which have the enhanced strength, durability and memory aspects of polyester, while retaining many of the desired qualities of the natural fiber with which the polyester is blended.
As with any fiber, the particular polyester fiber from which any given fabric is formed must have properties suitable for manufacture, finishing, and end use of that fabric. Typical applications include ring, open-end, and airjet spinning, either with or without a blended natural fiber, weaving or knitting, dyeing, and finishing. In addition, it has long been known that synthetic fibers such as polyester which are initially formed as extruded linear filaments, will exhibit more of the properties of natural fibers such as wool or cotton if they are treated in some manner which changes the linear filament into some other shape. Such treatments are referred to generally as texturizing, and can include false twisting, crimping, and certain chemical treatments.
In a homopolymeric state, polyester exhibits good strength characteristics. Typical measured characteristics include tenacity, which is generally expressed as the grams per denier required to break a filament, and the modulus, which refers to the filament strength at a specified elongation ("SASE"). Tenacity and modulus are also referred to together as the tensile characteristics or "tensiles" of a given fiber. In relatively pure homopolymeric polyester, the tenacity will generally range from about 3.5 to about 8 grams per denier, but the majority of polyester has a tenacity of 6 or more grams per denier. Only about 5 percent of polyester is made with a tenacity of 4.0 or less.
In many applications, of course, it is desirable that the textile fabric be available in a variety of colors, accomplished by a dyeing step. Substantially pure polyester, however, is not as dyeable as most natural fibers, or as would otherwise be desired, and therefore must usually be dyed under conditions of high temperature, high pressure, or both, or at atmospheric conditions with or without the use of swelling agents commonly referred to as "carriers." Accordingly, various techniques have been developed for enhancing the dyeability of polyester.
One technique for enhancing the dyeability of polyester is the addition of various functional groups to the polymer to which dye molecules or particles such as pigments themselves attach more readily, either chemically or physically, depending upon the type of dyeing technique employed. Common types of additives include molecules with functional groups that tend to be more receptive to chemical reaction with dye molecules than is polyester. These often include carboxylic acids particularly dicarboxylic or other multifunctional acids), and organo metallic sulfate or sulfonate compounds.
Polyethylene glycol ("PEG") is another additive that has been shown to offer improved dyeing characteristics when incorporated with polyester into textile fibers. If other practical factors and necessities are ignored, adding increased amounts of PEG to polyester increases the dyeability of the resulting polymer. Nevertheless, there are a number of disadvantages associated with the application of polyethylene glycol to polyester using these prior techniques, particularly when the PEG is added in amounts of 5 to 6 percent or more by weight, amounts which some references indicate are necessary to obtain the desired enhanced dyeability. These disadvantages are not generally admitted in the prior art patents and literature, but are demonstrated to exist by the lack of known commercial textile processes which use fibers formed essentially solely from copolymers of polyester and polyethylene glycol. These shortcomings can be demonstrated, however, by those of ordinary skill in the art using appropriate evaluation of the prior technology.
Most notably, commercially available fibers formed from polyester-polyethylene glycol copolymers tend to exhibit improved dyeability at the expense of tensiles; improved dyeability at the expense of shrinkage; improved tensiles at the expense of shrinkage; poor light fastness; poor polymer color (whiteness and blueness); unfavorable process economies; and poor thermal stability.
An improvement in the use of polyethylene glycol is disclosed in U.S. Pat. No. 4,975,233 to Blaeser et al. for "Method of Producing and Enhanced Polyester Copolymer Fiber." The Blaeser '233 patent teaches that the dyeability and tensile properties of a polyester filament can be enhanced by incorporating between about 1 and 4 percent by weight of the polyethylene glycol, and thereafter drawing and heat setting the resulting filament. Blaeser '233 suggests heat setting temperatures of about 370.degree. F., fibers of about 1.0 dpf and rotor spinning rotor speeds of about 95,000 rpm.
In general, however, using polyethylene glycol to increase dye uptake still requires high pressure techniques (with their associated costs and environmental control aspects) and an undesirable reduction in yarn quality. Additionally, the heat setting steps that help stabilize some of the yarn properties tend to produce fiber and yarn properties that discourage disperse dye uptake. Moreover, because the presence of polyethylene glycol still tends to decrease fiber and yarn strength, deep dyed polyester yarns (or blended yarns) have not been produced at spinning speeds greater than those achieved by the Blaeser '233 technique.
Accordingly, present techniques for increasing the dyeability of polyester fibers, yarns and fabrics all tend to require certain compromises among physical properties, available spinning speeds, costs, and related other factors.